While diving under water, it is frequently desired to communicate with another diver. In the absence of expensive and sophisticated equipment, such communication typically involves hand signals. But in order to convey a message in this manner, it is first necessary to get the other diver's attention. This is frequently a problem if the other diver is not looking at the diver wishing to convey the message, or at least in his general direction, and this is particularly a problem in situations where visibility is limited.
When one diver wishes to get the attention of another, the first diver will typically reach down and unbuckle a knife carried adjacent the ankle or the like. Using the knife, the first diver will then rap on the air tank attached to his back to produce a percussive sound which is easily heard by any nearby diver. The percussive sound produced is also not natural, so that the other diver is alerted that someone is trying to get their attention. In addition, the other diver also gets some sense of the direction from which the sound originated.
While such a sound producing system is effective, it suffers from a number of disadvantages. For example, the reaching for and grasping of the knife is time consuming, so that if the attention of the other diver is required immediately (i.e., to see something which may swim away), it may be too late once this form of communication is accomplished. In addition, in unbuckling of the knife from its holster on the ankle, the knife is subject to being accidentally dropped. Dropping of the knife not only prevents communication, but the knife itself may be irretrievable or lost.